Host food quality and quantity differentially affect Ascogregarina barretti parasite burden, development and within-host competition in the mosquito Aedes triseriatus

Host condition depends in large part on the quality and quantity of available food and heavily influences the outcome of parasite infection. Although parasite fitness traits such as growth rate and size may depend on host condition, whether host food quality or quantity is more important to parasite fitness and within-host interactions is poorly understood. We provided individual mosquito hosts with a standard dose of a gregarine parasite and reared mosquitoes on two food types of different quality and two quantities. We measured host size, total parasite count and area, and average size of parasites within each treatment. Food quality significantly influenced the number of parasites in a host; hosts fed a low-quality diet were infected with more parasites than those provided a high-quality diet. In addition, we found evidence of within-host competition; there was a negative relationship between parasite size and count though this relationship was dependent on host food quality. Host food quantity significantly affected total parasite area and parasite size; lower food quantity resulted in smaller parasites and reduced overall parasite area inside the host. Thus both food quality and quantity have the potential to influence parasite fitness and population dynamics.

Worms at war: interspecific parasite competition and host resources alter trematode colony structure and fitness

SUMMARYParasites competing over limited host resources are faced with a tradeoff between reproductive success and host overexploitation jeopardizing survival. Surprisingly little is known about the outcome of such competitive scenarios, and we therefore aimed at elucidating interactions between the trematodes Himasthla elongata and Renicola roscovita coinfecting the periwinkle first intermediate host. The results show that the success of Himasthla colonies (rediae) in terms of cercarial emission is unaffected by Renicola competition (sporocysts), whereas deteriating host condition decreases fitness. Furthermore, double infection has no bearing on Himasthla’s colony size but elevated the proportion of non-reproductive rediae that play a decisive role in colony defence. Opposite, the development of the Renicola colony (size/maturity), and in turn fitness, is markedly reduced in presence of Himasthla, whereas the nutritional state of the host appears less important. Hence, the intramolluscan competition between Himasthla and Renicola is asymmetrical, Himasthla being the superior competitor. Himasthla not only adjusts its virulence according to the hosts immediate nutritional state, it also nullifies the negative impact of a heterospecific competitor on own fitness. The latter is argued to follow in part from direct predation on the competitor, for which purpose more defensive non-reproductive rediae are strategically produced.

State-dependent parasitism by a facultative parasite of fruit flies

SUMMARYParasites can evolve phenotypically plastic strategies for transmission such that a single genotype can give rise to a range of phenotypes depending on the environmental condition. State-dependent plasticity in particular can arise from individual differences in the parasite's internal state or the condition of the host. Facultative parasites serve as ideal model systems for investigating state-dependent plasticity because individuals can exhibit two life history strategies (free-living or parasitic) depending on the environment. Here, we experimentally show that the ectoparasitic mite Macrocheles subbadius is more likely to parasitize a fruit fly host if the female mite is mated; furthermore, the propensity to infect increased with the level of starvation experienced by the mite. Host condition also played an important role; hosts infected with moderate mite loads were more likely to gain additional infections in pairwise choice tests than uninfected flies. We also found that mites preferentially infected flies subjected to mechanical injury over uninjured flies. These results suggest that a facultative parasite's propensity to infect a host (i.e. switch from a free-living strategy) depends on both the parasite's internal state and host condition. Parasites often live in highly variable and changing environments, an infection strategy that is plastic is likely to be adaptive.

Conditional persistence and tolerance characterize endoparasite–colonial host interactions

SUMMARYColonial hosts offer unique opportunities for exploitation by endoparasites resulting from extensive clonal propagation, but these interactions are poorly understood. The freshwater bryozoan, Fredericella sultana, and the myxozoan, Tetracapsuloides bryosalmonae, present an appropriate model system for examining such interactions. F. sultana propagates mainly asexually, through colony fragmentation and dormant propagules (statoblasts). Our study examines how T. bryosalmonae exploits the multiple transmission routes offered by the propagation of F. sultana, evaluates the effects of such transmission on its bryozoan host, and tests the hypothesis that poor host condition provokes T. bryosalmonae to bail out of a resource that may soon be unsustainable, demonstrating terminal investment. We show that infections are present in substantial proportions of colony fragments and statoblasts over space and time and that moderate infection levels promote statoblast hatching and hence effective fecundity. We also found evidence for terminal investment, with host starvation inducing the development of transmission stages. Our results contribute to a growing picture that interactions of T. bryosalmonae and F. sultana are generally characterized by parasite persistence, facilitated by multiple transmission pathways and host condition-dependent developmental cycling, and host tolerance, promoted by effective fecundity effects and an inherent capacity for renewed growth and clonal replication.

Solo Schistocephalus solidus tapeworms are nasty

SUMMARYTrophically transmitted parasites must trade-off own growth on one hand and energy drain from the intermediate host on the other hand, since killing the host before transmission to the next host is a dead end for both parasites and hosts. This challenge becomes especially intriguing when multiple parasites find themselves within the same individual host. The tapeworm Schistocephalus solidus may gain more than 98% of its final body mass within few months infecting its three-spined stickleback (Gasterosteus aculeatus) intermediate host. During these months the tapeworms may achieve a mass even larger than its host. We studied virulence of single and multiple infections of S. solidus, by comparing body condition of wild stickleback hosts in two perennial stickleback populations located at high latitudes, and each population was studied in two different years. Our results demonstrated multiple compared with single infections to be a highly significant predictor of the condition of stickleback hosts, with multiple-infected hosts having relatively higher body condition. However, this applied only after adjusting for parasite mass, which was another significant predictor for host condition. Thus, our results suggested that, at a given parasite mass, S. solidus was more harmful towards their host's body condition in single compared with multiple infections.